I want to make a function called 'load' which imports definitions of functions from another file. I know how to import modules, but in my program I want the definitions of the functions to change depending on which module is 'loaded' with this new function. Is there a way to do this? Is there a better way to write my program so that this is not necessary?

I think it's type signature would look something like:
load :: String -> IO ()
where the string is the name of the module to be loaded (and the module is in the same directory).

Edit: Thanks for all the replies. Most people agree that this is not the best way to do what I want. Instead, is there a way to declare a global variable from within an I/O program. That is, I want it so that if I type (function "thing") into a function of type String -> IO(), I can still type 'thing' into GHCi to get the value assigned to it... Any suggestions?

Can you give an example of where you think this would be useful? I have a feeling it's not possible (or at least easy), but I'm sure there's something better that will solve your problem. I'm just not sure what the actual problem you are facing is.
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Adam WagnerJan 7 '12 at 6:31

3 Answers
3

There is almost certainly a better way to write your program so that this is not necessary. It's hard to say what without knowing more details about your situation, though. You could, for instance, represent the generic interface each module implements as a data-type, and have each module export a value of that type with the implementation.

Basically, the set of loaded modules is a static, compile-time property, so it makes no sense to want your program's behaviour to change based on its contents. Are you trying to write a library? Your users probably won't appreciate it doing such evil magic to their import lists :) (And it probably isn't possible without Template Haskell in that case, anyway.)

The exception is if you're trying to implement a Haskell tool (e.g. REPL, IDE, etc.) or trying to do plugins; i.e. dynamically-loaded modules of Haskell source code to integrate into your Haskell program. The first thing to try for those should be hint, but you may find you need something more advanced; in that case, the GHC API is probably your best bet. plugins used to be the de-facto standard in this area, but it doesn't seem to compile with GHC 7; you might want to check out direct-plugins, a simplified implementation of a similar interface that does.

mueval might be relevant; it's designed for executing short (one-line) snippets of Haskell code in a safe sandbox, as used by lambdabot.

Unless you're building a Haskell IDE or something like that, you most likely don't need this (^1).

But, in the case you do, there is always the hint-package, which allows you to embed a haskell interpreter into your program. This allows you to both load haskell modules and to convert strings into haskell values at runtime. There is a nice example of how to use it here

^1: If you're looking for a way to make things polymorphic, i.e. changing some, but not all definitions of in your code, you're probably looking for typeclasses.